My 93 4x4 manual trans pickup with 3.0 3vze engine 298k miles) just started something strange.
after sitting for a few hours, it will sometimes run very rough but not completely stall. It seems to be missing on the same cylinder Or two because it is not bucking or surging, just running with noticeably less power. After a minute or so, it smooths out and runs perfectly at all speeds for as long as I keep it going. Even short stops are ok. But an overnight stop or a few hours to cool off and it does it again.
I can’t say I noticed this until yesterday but my speedometer is also not registering while it is missing. As soon as power comes back, the speedo jumps up and never falters again.
what would cause those two phenomena to happen in sync?

 

first guess....speed sensor. i'm not all familiar with m/t's on the 3.0, but when my a/t acted up liked that the sensor was the problem

 

ECU will store a code if the vss signal fails.

It's probably a ground issue.

 

If stud has 4wd, he has two speed sensors. VSS2, before the transfer case, is what drives code 42. VSS1, after the transfer case, drives the electronic speedometer.

When my VSS1 failed, it did not set the CEL. The speedometer would work, then drop to zero for 1 to many seconds, then pop back up. So my WAG is with thook.